1. Technical Field
The present invention relates to holographic memories, holographic storage systems and holographic processors.
2. Background Art
The traditional advantages of 3-D holographic memories are high storage density and parallel access capability. These features were recognized in the early 1960's and serious efforts towards the practical implementation of such memories were undertaken. Unfortunately, these efforts did not produce commercially viable memories. In recent years there has been a resurgence of interest in 3-D optical storage due to a considerable improvement in the understanding and availability of storage media, a dramatic improvement in optoelectronic components in general, and most importantly, the emergence of applications, such as image processing, neural networks, and data bases where the capabilities of these memories can be effectively utilized. This recent activity has culminated in the storage of 10.sup.4 320.times.220 pixel holograms in a volume roughly equal to 2 cm.sup.3. If spatial light modulators with 1 million pixels are used, then the storage density achievable today is in excess of 10.sup.9 bits per cm.sup.3.
Volume holograms are usually recorded using angular, wavelength, phase code, and spatial multiplexing. In addition, peristrophic multiplexing, a holographic technique that applies to either thin or thick (3-D) media, was recently introduced. Any of these methods, or certain combinations of them, can be used to multiplex holograms for holographic storage devices. All of these methods employ a reference beam consisting of a single plane wave, which may have a phase code imprinted on the wavefront.
Spherical waves can also be used as a reference beam. A volume hologram recorded with a spherical wave reference becomes Bragg mismatched when it is translated with respect to the readout beam by an amount denoted as shift selectivity .delta.. This distance is usually on the order of a few microns up to a few hundreds of microns, depending on the geometry, the material thickness and the distance of the focus of the spherical wave to the hologram. Multiplexing is realized by recording a new hologram after shifting by .delta.. This method is called shift multiplexing and is convenient in 3D holographic disk configuration, since disk rotation produces the required relative shift.
The novel method of shift multiplexing was disclosed in the co-pending U.S. patent application Ser. No. 08/389,890. The main property of the shift multiplexing method is that it uses non-planar wavefront for the reference beam, and as a result the Bragg mismatch required to superimpose multiplex holograms without significant crosstalk is produced by translation rather than angular or wavelength change. Shift multiplexing is applicable to any kind of volume holographic material.
In addition, in photorefractive materials photoexcitation effects during hologram readout, or even thermal excitation during dark storage, cause the holograms to decay with time, and eventually to be erased. The problem of non-destructive readout was recently addressed in "Non-volatile storage in photorefractive crystals", by D. Psaltis et al., Optics Letters 19, 210, 1994. This reference disclosed the "two lambda method" in which the hologram is recorded at a wavelength where the photorefractive material is sensitive, e.g. 488 nm, but read out at a wavelength where the absorption is relatively low, e.g. 633 nm. However, these references did not provide a solution for storage in photorefractives using shift multiplexing.
It is the objective of this invention to apply a two-lambda technique to shift multiplexed holograms so that non-volatile readout of shift-multiplexed photorefractive disks can be achieved. In this way the advantages of shift multiplexing can be combined with long-term storage in a read/write memory configuration.